The invention relates to methods and systems for facilitating a multi-mode multi-pilot hard handoff within a wireless communications system.
Where previously all CDMA mobile stations were single CDMA band class, for example 1900 MHz CDMA, multi-mode CDMA mobile stations are now becoming common. For example, mobile stations are now available with both 800 MHz AMPS and 800 MHz CDMA, and with both 800 MHz AMPS and 1900 MHz CDMA. There are also mobile stations with all three modes mentioned above, namely 800 MHz CDMA, 1900 MHz C and 800 MHz AMPS. Mobile stations which handle other modes/band classes and combinations of modes/band classes may also exist and almost certainly will continue to be developed.
Of course, such mobile stations are frequently involved in handoffs within a given mode, but they are also involved in handoffs between modes. Multi-Mode hard handoffs provide the capability of performing a hard handoff from a cell having one mode to a cell having another mode.
For example, depending upon the capability of a given mobile station, it may be necessary to perform a hard handoff:
from a 1900 MHz CDMA system to an 800 MHz CDMA system;
from an 800 MHz CDMA system to a 1900 MHz CDMA system;
from an 800 MHz CDMA system to an 800 MHz AMPS system; or
from a 1900 MHz CDMA system to an 800 MHz AMPS system.
Multi-mode hard handoff functionality is needed, for example, for tri-mode networks, i.e., 800 MHz AMPS networks with both 1900 MHz CDMA and 800 MHz CDMA overlays. Knowing the capability of the phone becomes important since there will be a mix of mobile types (single, dual, tri-mode) in the network. Blindly handing a phone to a particular target system without taking the capability of the phone into account causes an unnecessary amount of dropped or degraded quality of calls in situations where a call could be maintained on a different system. For example, blindly handing all phones currently using 1900 MHz CDMA to 800 MHz CDMA, where 1900 MHz CDMA runs out of coverage, will cause dual-mode 1900 MHz CDMA/800 MHz AMPS phones to drop calls where cells may have been sustained by 800 MHz AMPS instead. Similarly, blindly handing all phones currently using 1900 MHz CDMA to 800 MHz AMPS, where 1900 MHz CDMA runs out of coverage, will cause tri-mode 1900 MHz CDMA/800 MHz CDMA/800 MHz AMPS phone to experience degraded quality where the quality of the call may have been sustained by handing off to 800 MHz CDMA instead. In a tri-mode network where subscribers have single-mode, dual-mode, or tri-mode phones, the client does not want calls to be necessarily dropped or handed down to AMPS when calls could otherwise be maintained on another CDMA system.
With existing multi-mode handoff techniques, when a source base station (or base stations if the mobile station is currently in soft handoff) and mobile station agree a handoff is necessary, a hard handoff to one or more new cells is attempted. This may involve various messages being passed to a first mobile switching centre, and possibly further on to a second mobile switching centre servicing a different set of basestations than that of the first mobile switching centre, and ultimately to one or more target base stations serving the new cells. In the event that no successful allocation of target resources results from this attempt, this fact is propagated all the way from the target base station back to the source base station which then may instigate an attempt to handoff to one or more different cells. This is notwithstanding the fact that a subsequent attempt may be routed through again to the second mobile switching centre, and possibly even to the same target base station.
Unfortunately, the time taken to propagate through from the source base station to the target base station and back on each attempt can be quite lengthy with the result that calls may be frequently dropped.
It is an object of the invention to obviate or mitigate one or more of the above identified disadvantages.
According to an embodiment of the invention, a method is provided for a handoff intermediary to participate in handing off a mobile station. The handoff intermediary generates a first group of cell identifiers of potential cells for handoff in the event that the handoff intermediary is a source base station. In the event that it is some other handoff intermediary which is source-to-target of the source basestation, it would receive a group of cell identifiers. The handoff intermediary sends a handoff intermediary in the source-to-target direction a second group of cell identifiers, the second group being a subset of the first group which identifies all cells in the group serviced through the handoff intermediary in the source-to-target direction. In so doing, the source-to-target handoff intermediary is provided with information with which it can participate in multiple attempts at handing off the mobile station to cells identified in the second group of cell identifiers without sending a handoff failure back to the previous handoff intermediary after each failed attempt. Preferably, the first group of cell identifiers includes cell identifiers for cells collectively providing at least two different capabilities. The group may include one or more cell attributes for each cell identifier, the cell attributes of a given cell identifier comprising at least a cell capability identifier. The group includes one or more reference target cells for each mode/capability represented in the group and may include additional target cells for each reference target cell. More specifically, there may be one reference target cell for each frequency/carrier for a given mode. Bach lone reference target cell, and each reference target cell together with its associated additional target cells is a potential reference target cell or group of target cells for hard handoff. If a given reference target cell has associated target cells then the mobile station would be in soft handoff after completion of the hard handoff. Advantageously, source-to-target handoff intermediaries do not need to generate failure messages to handoff intermediaries in the target-to-source direction until they have exhausted their options for handoff.
This invention facilitates inter-system multi-mode multi-pilot hard handoffs by passing for each mode or capability of the mobile station, reference target cell information and additional target cell information from the serving system to the target system. In a particular example, this is passed in various forms from a source base station to a serving mobile switching centre, from the serving mobile switching centre to a target mobile switching centre, and from the target mobile switching centre to a target base station. This target cell information is used by the target system to perform the handoff and may also be used for other functions such as handoff attempts, load balancing, and target screening by taking into account the mobile station""s capability.
Reference target cell information consists of a reference target cell designation and reference target cell attributes including at least an identification of the made of the target cell.
When multiple systems are involved in the handoff of a mobile station, target cell information is passed from the serving system to the target system relating to all capabilities which are both provided by the target system and handled by the mobile station. Preferably, this invention enhances the ANSI-41 add CDG IOS A-Interface specifications to include reference target cell information and their attributes.
Advantageously, the invention increases the reliability of inter-system CDMA multi-mode multi-pilot hard handoffs by expediting handoff attempts and handoff retries at the target system.